The present invention relates to a methods for checking a device having a plurality of light emitting elements. More precisely, it relates to methods for checking and regulating the emission characteristics of elements in light emitting arrays used for electronic printers.
In recent printing machines, copying machines or display equipment a plurality of light emitting elements arranged in an array. For example, in an electronic printer applying electrophotography, about 2,800 light emitting elements are aligned with a pitch of 250 units per inch on a line of about 30 cm long. These light emitting elements might be laser diodes, light emitting diodes (LED) or a liquid crystal shutter (LCS) array. Among of them, the LED array is becoming the primary choice for this application due to its low cost and quick response.
In the following disclosure of the present invention, the description refers to an LED array applied in electronic printers. Application of the invention, however, is not limited to the linear array of LEDs for electronic printers. It may be extended to any device which is provided with a plurality of light emitting elements arranged in a two dimensional matrix such as a planer solid state display device, light emitting elements also may be of any kind.
In an LED array used for electronic printers a plurality of LEDs are arranged on a line. Each LED has a size corresponding to the size of a picture element with a pitch for providing necessary resolution of the image. It is important to check the light emitting condition of these LEDs. If one LED among the units in an array is broken or degraded, the image obtained from such an LED array will include a line defect.
Further, it is necessary to bring the brightness of each LED into a predetermined tolerance in order to obtain a uniform output image. Therefore, in a practical electronic printer, the LED array is checked and the emission property of each LED element is adjusted before the array is installed into the printers. In some electronic printer, there is provided a control circuit for controlling the amplitude or pulse width of the signals during each of the LEDs, so as to compensate the tolerances of their brightness. This is done by measuring the light emitting characteristics of each of the LEDs one by one, and memorizing their characteristics in a memory device provided in the printer device. The amplifier for driving the LEDs is adjusted to vary its output pulse width or current amplitude referring to the memory. Such measurement and memorizing operation are done before the LED array is built into the printer device. More detail is disclosed in for example "High Printing Quality LED Printhead" by I. Abiko et al., Jour. of Image Technology, pp. 296-299, Vol. 12, Number 5, Oct. 1986, Society of Photographic Scientists and Engineers, or in U.S. Pat. No. 4,455,562, Jun. 19, 1984, by D. T. Dolan "Control of a light Emitting Diode Array".
Even though such correction is done, there still remains a problem. At the beginning of the operation, the output image may be fine, but the image quality as a whole degrades or fails if the LED elements degrade or fail during the operation. Since there is no way to forecast the failure of each LED element, the LED array is replaced or repaired after some defects appear in the output image. Or in some cases when defects should be avoided completely, the LED array is replaced before its total operation time exceeds the average life.
If a method is developed to check each of the light emitting elements during their operation, or during a short time interval between the operation of the electronic printer, for example during a time period of feeding a paper for a next page, extensive improvements can be expected in the quality, reliability, and maintainability of the device using such light emitting elements.